516 



CHORD AT A. 



it is gradually replaced Ly the verteljne arising around it. It is of 

 entodernial origin (fig. 9), arising as a longitudinal band of the 

 ei)itlielium of the archenteron (/, cli), and, becoming cut off, 

 comes to lie in the long axis of the body between digestive tract 

 and nervous system (//, ///) ; here it forms a cylindrical rod con- 

 sisting of a connective tissue which, as already said, resemljles 

 plant tissues because of the vesicular nature of its cells (fig. 38). 

 In transverse section (fig. 555) the chorda is surrounded by 

 three layers, internally by a fibrous noto- 

 cliordal sheath, then an elastic layer (not 

 always present), the elastica externa, so 

 ,^,., called because an elastica interna is some- 



■^'—#^#12^ ^ times present inside the notochordal 



mA ^,, sheath; and lastly a skeletogenous layer 



/-> {SS), also called the outer notochordal 



'^ "^^^^^^i // sheath. This last is a mesodermal con- 



, , \-iWioS9....-' r nective-tissue layer and is therefore cou- 



'''^f/^-yJ - — f'>- nected with the other connective-tissue 



/'---J|^==^^| jy sheaths which surround muscles, nerves, 



etc., and deserves special mention because 

 - /z in it the cartilages and bones arise from 

 which the vertebra and skall are formed. 

 Cells from it can penetrate the notochor- 

 FiG. .ws.-Transverse section dal sheath, converting it into fibrous car- 



of axial skeleton of Vc- it-,, i- ■ 



trnmiimn. (From Wieders- tiiage, tlius enabling it to participate m 



heim.) C, notoohord ; Cs, , , , 



notochordal sheath; K, the formation ot the Vertebra?. 



elastica externa ; F, fatty c>- ±i x i t n -^ i i.i 



tissue ; M, spinal cord ; p. Since the iiotochord and its slieaths 



its meninges: Oh, upper i x- i ■ n xi 4. ■ j.* i-i 



process of skeletogenous are elastic and give under the strain ot tlie 

 ^S! 1^,iowlJ'^S^i muscles, they are unsegmented. The seg- 

 of skeletogenous tissue. mentation of the axial skeleton begins with 

 the appearance of firmer tissue in cartilage and bone. Then there 

 is a separation of successive parts, and with this the gradual forma- 

 tion of vertebral column and skull. For both there is a con- 

 nected series of developments, if studied with reference to the 

 ontogenetic jirocesses or in the comparative manner from the 

 lower to the higher forms. 



The first parts of the vertel)ral column to appear are the upper 

 and lower (figs. 555, 55G), or neural and hcemal ardies. These 

 consist of paired parts in the skeletogenous layer which abut 

 against the notochord, and which are usually a pair to the somite, 

 although occasionally two or more pairs, the arches proper and 

 the intercalaria, may occur. The neural arches (arcus vertebra? 



